Pollution and pollution control have become increasingly more important in recent years, not only because people are more sensitive to their environment, but also because the amount of pollution has increased markedly in some instances. Pollution represents a potential health hazard and a deterioration in the quality and beauty of our surroundings. It is one of the undesired consequences of our society's rising production, our modern civilization, and our increased use of natural resources.
Water is mankind's most valuable resource. Any use of water nearly always results in a decrease in the quality of the water by the addition of pollutants thereto. These pollutants can comprise dissolved inorganic and organic chemicals, inorganic and organic particulate material, and absorbed gases. In general, several methods are available for removing pollutants. These methods include such processes as gravity sedimentation, flotation, filtration, biological oxidation, ion exchange, activated adsorption, reverse osmosis, electrodialysis, distillation, and chemical precipitation. Several soluble inorganic pollutants present a problem that generally is not removed by gravity sedimentation, flotation, filtration, or biological oxidation.
Stringent standards for the maximum level of pollutants in water to be used for drinking or released into groundwater systems are being promulgaed by various federal and state agencies. For example, the maximum concentration level for selenium in drinking water has been set at less than 0.01 milligram per liter, and some jurisdictions have proposed a selenium standard of 0.05 milligrams per liter for discharge into the ground water system of the jurisdiction.
Another major problem with respect to pollution is agricultrual drainage water from areas such as the San Joaquin Valley. This Valley has approximately 1.1 million acres of extremely productive land under irrigation. The land is generally low-lying and requires drainage in order to avoid high salinity in crop root zones. Buried plastic pipe is commonly used to remove applied irrigation water from the root zone of filed crops. The resulting drainage water is high in selenium and therefore represents a serious disposal problem. Environmental concern has led to a suspension of construction work, a closure order on a large reservoir, and an extensive program of drain plugging. With the plugging of the drains, the water table is beginning to rise, causing concern for growers in the Valley.
Selenium is one of the heavy metals that is most difficult to remove from water. Although commercial elemental selenium is relatively inert, a number of selenium compounds, such as the reactive selenides, the gaseous, volatile, and soluble compounds, and particularly hydrogen selenide, the halides and oxyhalides, and the organics have been found to be particularly poisonous to humans and animals. These compounds can enter the body through the lungs, the skin, or through ingestion of polluted water, and may affect the body organs.
Selenium is primarily present in groundwater in the selenate (SeO.sub.4 =) form, which is particularly hard to absorb on conventional adsorbents which are used to treat water for purification purposes. Therefore, when water is treated for removal of pollutants, often the selenium is not removed by the conventional treatments.
Methods are available for the purification of water, but in general the known methods are not suited for use on a large scale with brackish water containing selenium and a range of other contaminants. Reverse osmosis is costly, and so also is ion exchange. Since the initial aim in treating drainage water is to render it suitable for discharge, an economic process is required.
Terajima et al., U.S. Pat. No. 3,970,553, disclose a method for removing heavy metal ions such as selenium by using a heavy metal adsorbent composed predominantly of nitrohumic acid. The nitrohumic acid can be prepared by oxidation of coal with nitric acid.
Baker, in U.S. Pat. Nos. 4,790,945 and 4,792,405, discloses a method for removing hydrogen selenide from a liquid stream by treating the liquid with a disulfide or polysulfide to oxidize the selenide to elemental selenium.
Downing et al., in U.S. Pat. No. 4,725,357, disclose a method for removing dissolved selenium from water by treating the water in a reactor containing microbial biomass to cause the hexavalent selenium to be converted to forms of selenium which can be readily removed from the water.
DuFresne, in U.S. Pat. No. 4,096,064, discloses a system for removing toxic heavy metals such as selenium from drinking water by electrochemical replacement while the water flows through a tandem bed of activated zinc and then magnesium. The zinc is activated by contact with a noble metal salt.
Baldwin et al., in U.S. Pat. No. 4,405,464, disclose a process for removing selenium from aqueous systems by mixing the aqueous solution with a quantity of metallic iron to a lower oxidation state. The selenium ions are reduced to at least the Se(IV) oxidation state, and the dissolved metallic iron hydrolyzes to form an iron oxide that precipitates. The precipitated material is separated from the aqueous solution to provide a solution having a lower concentration of selenium ions. At least a portion of the selenium is eventually reduced to the elemental state, which can then be separated from the aqueous solution.
Baldwin et al., in U.S. Pat. No. 4,519,913, disclose a process for reducing the concentration of water soluble ionic selenium in aqueous waste water. The waste solution is passed through a treatment zone containing a porous matrix on which are retained populations of at least one bacterium of the genus Clostridium. The water soluble ionic selenium species is converted to water insoluble selenium metal, which is retained on a porous matrix from which it can be recovered.
Sabherwal, in U.S. Pat. No. 4,530,765, discloses a method for preparing particulate materials for treating wastewater by calcining a charge of clay mineral, feldspar, zeolite, coal ash, fly ash, pozzolan, or volcanic ash. Although there is a disclosure that heavy metals can be removed from wastewater using this material, there is no disclosure that selenium can be removed from wastewater by using this material.
None of the above patents discloses the use of an inexpensive material that quickly adsorb large amounts of selenium.